This paper introduces two time-frequency analysis schemes based on nonlinear semiconductor laser dynamics reported recently by us. The key to the two schemes is the nonlinear period-one (P1) semiconductor laser dynamics. By injecting an optical signal with linearly varying intensity over time into a semiconductor laser, a wideband frequency-sweep optical signal is generated via the P1 oscillation. Subsequently, the wideband frequency-sweep optical signal is modulated by the Signal Under Test (SUT), generating multiple frequency-sweep optical sidebands that are directly associated with the SUT frequency. Then, an optical narrowband filter is used to process these optical sidebands to implement the frequency-to-time mapping and the final time-frequency analysis. The narrow transmission peak in the notch of a phase-shifter fiber Bragg grating and the Stimulated Brillouin Scattering (SBS) gain spectrum are used as the optical narrowband filter, respectively. The former features a simple system structure and easy implementation but its bandwidth is hard to adjust. The latter, because its bandwidth is easy to manipulate, can meet the needs of performance optimization under different sweep chirp rates. To solve the problem of limited measurement resolution caused by the instability of P1 oscillation, an optoelectronic feedback loop is employed to stabilize the P1 oscillation to improve the stability and performance of the system. Furthermore, the nonlinearity of the generated frequency-sweep optical signal is compensated through pre-compensation or post-compensation. Using the proposed system, the time-frequency information of SUTs in a 4-GHz bandwidth is acquired.
Spectrally Efficient Frequency-Division Multiplexing (SEFDM) is a promising solution to increase communication spectral efficiency, which can pack even more sub-carriers than the orthogonal frequency-division multiplexing in a given bandwidth. However, the cost is the introduction of Inter-Carrier Interference (ICI), increasing the difficulty of signal reception and demodulation. When SEFDM signals are incorporated into a microwave photonic link, in addition to ICI, the nonlinear interference introduced by the nonlinearity of the microwave photonic link should also be considered. In this work, an iterative algorithm for microwave photonic SEFDM transmission systems is proposed to compensate for the inherent ICI of the SEFDM signal and reduce the third-order Intermodulation Distortion (IMD3) introduced by the microwave photonic transmission link. In the digital algorithm, the received 16 Quadrature-Amplitude Modulation (QAM) SEFDM signal experiences several iterations, and in each iteration, the input SEFDM signal is modified, demodulated, and Forward Error Correction (FEC) decoded into a bit sequence, which is remapped to QAM symbols to reconstruct the interference signals for canceling the distortion. Experimental results show that 16-QAM SEFDM signals with a bandwidth compression factor of 0.85 and high nonlinearity are successfully recovered from a microwave photonic link. The proposed method integrates the demodulation of SEFDM signals with the elimination of IMD3. Due to the employment of the FEC, compared with the traditional iterative ICI compensation (IIC) algorithm only for SEFDM signal demodulation, the signal demodulation capability is improved, and the improvement of signal demodulation capability also provides great help for the elimination of IMD3.
In this work, time-frequency transform systems reported recently by us, including short-time Fourier transform and wavelet-like transform, are introduced. A periodic fast frequency-sweep electrical signal is used to scan the Signal Under Test (SUT). As long as the sweep is fast enough, the SUT in a single sweep period can be seen as a stationary signal and its frequency in this period can be obtained by mapping it to low-frequency pulses using filtering and frequency-to-time-mapping technique. After obtaining the frequency information of the SUT in each sweep period, the time-frequency information of the SUT can be obtained by combining the frequency information in each sweep period. The proposed method converts the time-frequency analysis of broadband signals into the analysis of low-speed electrical pulses, greatly increasing the real-time performance of the system and not relying on dispersive mediums compared to existing photonics-assisted solutions. The method for improving the system performance is also discussed by introducing filter bandwidth manipulation technology. It is found that for a given sweep speed, a proper filter bandwidth can be found to minimize the width of the electrical pulses and optimize the system frequency resolution. The photonics-assisted analog time-frequency transform method introduced in this work has a broad application prospect in the efficient and real-time acquisition of two-dimensional time-frequency information of the electromagnetic spectrum.
Photonic random demodulator (RD) is a widely studied scheme to realize photonics-assisted compressive sensing (CS). Since CS via an RD requires a pseudo-random binary sequence (PRBS) bit rate that is at least twice the signal’s Nyquist rate, the generation of the ideal PRBS in practical systems requires a very large signal bandwidth and is difficult to achieve, leading to poor mixing results from the RD and limited performance of the photonics-assisted CS system. To solve this problem, in this work, we propose a method to compensate for the poor mixing results by using a complementary PRBS and a parallel optical path, thus achieving better CS results in the case of limited PRBS bandwidth. The feasibility of the proposed method is verified by simulation. When only a 4-Gbps PRBS with a bandwidth limited to 0 to 6 GHz is used and the signal to be sampled is a multi-tone signal lower than 1.6 GHz, it is difficult to reconstruct the multi-tone signal or the quality of the reconstructed signal is poor when the starting point is randomly selected for downsampling due to the non-ideal PRBS. When the complementary PRBS is also limited to 0 to 6 GHz, the quality of the reconstructed signal is also improved in most cases. The method proposed in this work provides a good solution for the realization of CS using photonics-assisted RD in real systems, which can reduce the influence of non-ideal PRBS on signal recovery without expanding the bandwidth of the used PRBS.
A multi-band microwave vector signal generation and transmission scheme is proposed for mobile fronthaul networks based on dense wavelength division multiplexing (DWDM) and optical tandem single-sideband (OTSSB) modulation. At the distributed cell, two adjacent DWDM channels are divided into a group and both the frequency spacing of the adjacent optical carriers and the adjacent DWDM channels are 50 GHz in the system. A pre-distortion circuit is used for mitigating the modulation non-linearity. At an active antenna unit, when both two DWDM channels in a group are selected, six vector signals centered at different frequencies from 3.5 to 60 GHz are generated. When only one of the DWDM channels is selected, two vector signals centered at 16.5 and 33 GHz or two vector signals centered at 3.5 and 10 GHz can be generated. A simulation is performed to verify the proposed system and its performance by employing 16 quadrature amplitude modulation signals. In the simulation, the error vector magnitudes (EVMs) for different frequency vector signals are around 1% in the back-to-back case. When a section of 25-km single-mode fiber is inserted, the EVMs are still better than 5%.
KEYWORDS: Polarization, Modulators, Silicon carbide, Signal detection, Radio over Fiber, Radio optics, Single mode fibers, Phase shifts, Modulation, Dispersion
A photonic approach for radio-frequency (RF) self-interference cancellation (SIC) incorporated into an in-band full-duplex radio-over-fiber system is proposed. A dual-polarization binary phase-shift keying modulator is used for polarization multiplexing at the central office (CO). A local oscillator signal and an intermediate-frequency signal carrying the downlink data are single-sideband modulated on the two polarization directions of the modulator, respectively. The optical signal is then transmitted to the remote unit, where the optical signals in the two polarization directions are split into two parts. One part is detected to generate the up-converted downlink RF signal, and the other part is re-modulated by the uplink RF signal and the self-interference, which is then transmitted back to the CO for the signal down-conversion and SIC via the optical domain signal adjustment and balanced detection. The functions of SIC, frequency up-conversion, down-conversion, and fiber transmission with dispersion immunity are all incorporated into the system. An experiment is performed. Cancellation depths of more than 39 dB for the single-tone signal and more than 20 dB for the 20-Mbaud 16 quadrature amplitude modulation signal are achieved in the back-to-back case. The performance of the system does not have a significant decline when a section of 4.1-km optical fiber is incorporated.
A high-stable photonics-based linearly frequency-modulated (LFM) signal generation approach for radar applications is proposed and experimentally demonstrated. A broadband LFM signal is generated by a photonic frequency quadrupler consisting of a Mach-Zehnder modulator and a fiber Bragg grating (FBG)-based optical notch filter. The stability of the generated signal is improved by using modulator bias control, laser wavelength control, and FBG temperature control. An experiment is performed. A Ka-band LFM signal with an instantaneous bandwidth of 4 GHz is generated. Meanwhile, inverse synthetic aperture radar (ISAR) imaging and target distance measurement are implemented by using a generated 1-GHz bandwidth LFM signal. The LFM signal remains stable during the four-hour experiment.
A photonic-assisted image rejection mixer is proposed based on Hilbert transform in the optical domain using a phase-shifted fiber Bragg grating (PS-FBG). The image signal and the desired RF signal are converted to the optical domain by carrier-suppressed single-sideband modulation, which is then split into two parts. One part is Hilbert transformed by a specially designed PS-FBG and a 90-deg optical phase shifter and then combined with the other part to directly reject the image signal in the optical domain. The image-free optical signal is downconverted to an intermediate frequency signal by combining it with an optical local oscillator signal and then detecting them in a photodetector. The image rejection capability of the system is analyzed, and an image rejection ratio of 68.0 or 58.6 dB is achieved when the bandwidth of the image signal is 20 or 60 MHz, respectively, while having an acceptable influence on the desired RF signal. Quadrature phase-shift keying (QPSK) signals, 8 phase-shift keying (8PSK) signals, and 16 quadrature amplitude modulation (16-QAM) signals are used to evaluate the image rejection performance. When the bandwidth of the image signal is 18.75 MHz, the error vector magnitudes of the downconverted QPSK, 8PSK, and 16-QAM signals with the same power, modulation format, and bandwidth are 2.5%, 2.5%, and 2.6%, respectively.
A photonic-based approach for multifunctional microwave signal generation and processing is demonstrated based on equivalent phase modulation. The key component of the system is a dual-polarization quadrature phase-shift keying (DPQPSK) modulator. One dual-parallel Mach-Zehnder modulator (DP-MZM) in the DP-QPSK modulator is biased to function as an equivalent phase modulator (e-PM), while the other DP-MZM is biased as a carrier-suppressed singlesideband (CS-SSB) modulator. The two optical signals from the two DP-MZMs are combined and detected in a photodetector. With different driving signals applied to the two DP-MZMs, different functions can be achieved. When the e-PM is driven by a direct current signal to phase shift the optical carrier, and the CS-SSB modulator is to generate a first-order optical sideband of the driving RF signal, a wideband microwave phase shifter is implemented, which can introduce arbitrary phase shift to the electrical driven signal applied to the CS-SSB modulator. Under the above condition, if the CS-SSB modulator is to generator a first-order and an opposite third-order optical sidebands of the driving RF signal, a repetition rate tunable triangular and square waveform generation scheme can be realized. When the e-PM is driving by an electrical coding signal, and the CS-SSB modulator is to generate a first-order optical sideband of the driving RF signal, a reconfigurable pulse compression signal generator is achieved. Experimental verifications are made to demonstrate the multifunctional system, which has the potential to be used in a variety of microwave systems.
A photonic-based approach for microwave spectrum sensing is proposed based on optical carrier-suppressed single-sideband (CS-SSB) modulation and coherent detection. Two dual-parallel Mach-Zehnder modulators (DP-MZMs) function as two CS-SSB modulators. A local oscillator (LO) signal with its frequency swept at a fixed step of Δf is applied to one DP-MZM, while the microwave signal to be detected is applied to the other DP-MZM. The two CS-SSB modulated optical signals from the two DP-MZMs are sent to a coherent receiver, which consists of a 90° optical hybrid and two balanced photodetectors. The two outputs of the coherent receiver are combined by a 90° electrical hybrid and filtered by an electrical low-pass filter with a bandwidth of Δf. Different frequency components in the unknown microwave signal are all frequency downconverted to within Δf and the signals in different frequency bands appear in different time periods, which can be used for spectrum sensing by sampling and processing the IF signal. In addition, due to the balanced detection, the direct current components are suppressed, and the frequency-swept LO signal only detects the microwave signal on its left or right side, avoiding the interference from the image frequency. To verify the proposed technique, spectrum sensing in a frequency range from 1 to 20 GHz is demonstrated by simulation.
A photonic approach for simultaneous frequency down-conversion, self-interference cancellation, and image rejection for in-band full-duplex radio-over-fiber systems is proposed based on a dual-polarization quadrature phase-shift keying (DP-QPSK) modulator. The upper dual-parallel Mach–Zehnder modulator (DP-MZM) of the DP-QPSK modulator realizes the radio frequency (RF) self-interference cancellation and generates the upper and lower first-order optical sidebands of the desired RF signal, whereas the lower DP-MZM generates two optical sidebands of the local oscillator. The optical signals from the two DP-MZMs are combined, and the upper and lower optical sidebands are separated by a wavelength division multiplexer (WDM). The two outputs from the WDM are detected at two photodetectors for frequency down-conversion. By combining the two intermediate-frequency (IF) signals at a 90°hybrid coupler, the proposed system can also achieve image rejection. In addition, by changing the bias point of the lower DP-MZM, second-harmonic frequency down-conversion can be implemented. A simulation is performed. A QPSK-modulated RF signal from 5 to 44 GHz is successfully down-converted to 1-GHz IF signal with self-interference and image frequency cancelled, and the EVM of the received IF signal is lower than 15%.
KEYWORDS: Fiber Bragg gratings, Modulators, Modulation, Signal to noise ratio, Transmitters, Signal generators, Radio over Fiber, Radio optics, Signal attenuation, Signal detection
A photonic-assisted radio-frequency (RF) self-interference cancellation (SIC) scheme for in-band full-duplex radio-over-fiber system is proposed based on a dual-drive Mach–Zehnder modulator (DD-MZM) and a fiber Bragg grating (FBG). The received signal with an interference from the transmitter is applied to one arm of the DD-MZM, and the reference signal tapped from the transmitter is injected into another arm of the DD-MZM. By properly controlling the amplitude and phase of the reference signal, and adjusting the bias point of the DD-MZM, the RF interference can be cancelled in the optical domain. An FBG is used to convert the phase-modulated received signal to an intensity-modulated signal for information recovery. Since the self-interference is cancelled directly in the optical domain, the proposed SIC scheme is suitable for long-distance fiber transmission. A simulation is performed, where a cancellation depth of more than 50 dB is achieved. The proposed SIC system features good transmission capability and frequency tunability, which is only limited by the bandwidths of the DD-MZM and the FBG.
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